Low Capping Group Surface Density on Zinc Oxide Nanocrystals

Carolyn N. Valdez,† Alina M. Schimpf,† Daniel R. Gamelin,† and James M. Mayer†,‡,* †

Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States and ‡Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States

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Low Capping Group Surface Density on Zinc Oxide Nanocrystals

ABSTRACT The ligand shell of colloidal nanocrystals can dramatically affect

their stability and reaction chemistry. We present a methodology to quantify the dodecylamine (DDA) capping shell of colloidal zinc oxide nanocrystals in a nonpolar solvent. Using NMR spectroscopy, three different binding regimes are observed: strongly bound, weakly associated, and free in solution. The surface density of bound DDA is constant over a range of nanocrystal sizes, and is low compared to both predictions of the number of surface cations and maximum coverages of selfassembled monolayers. The density of strongly bound DDA ligands on the as-prepared ZnO NCs is 25% of the most conservative estimate of the maximum surface DDA density. Thus, these NCs do not resemble the common picture of a densely capped surface ligand layer. Annealing the ZnO NCs in molten DDA for 12 h at 160
C, which is thought to remove surface hydroxide groups, resulted in a decrease of the weakly associated DDA and an increase in the density of strongly bound DDA, to ca. 80% of the estimated density of a self-assembled monolayer on a flat ZnO surface. These findings suggest that as-prepared nanocrystal surfaces contain hydroxide groups (protons on the ZnO surfaces) that inhibit strong binding of DDA. KEYWORDS: capping ligands . colloidal semiconductors . zinc oxide . ligand shell . NMR spectroscopy of nanocrystals

C

olloidal semiconductor nanocrystals (NCs) are widely investigated for use in solar cells, light emitting devices, luminescent biological labels, and many other applications.1
3 NCs are unique materials, combining the tunable electronic structure and photochemical properties associated with bulk materials with the synthetic accessibility and spectroscopic convenience of small molecules.4,5 Most NC preparations in nonpolar solvents use capping ligands to control solubility, photoluminescence, and other properties.6,7 The capping ligand density of each system is an important parameter to a range of chemical and physical properties of NCs, such as the inner sphere reactivity of NCs with hole acceptors8 and proton-coupled electron transfer (PCET) acceptors,9,10 as well as interparticle electron transfer.11 Recent studies show that the capping ligands on semiconductor NCs are more complex and dynamic than the traditional image of a fairly dense shell.8
17 In work related to that reported here, Kahn et al. identified different types of dodecylamine (DDA) in the ligand shells of large ZnO NCs (prepared by a different synthetic method),15 but they did not quantify the number of VALDEZ ET AL.

capping ligands per NC. The literature of capping groups on other semiconductor NCs, especially CdSe, is much more extensive.13,16,18
20 In particular, Weiss et al. have reported that the density and dynamics of the capping groups strongly influence the chemical reactivity of chalcogenide NCs.8 Of the variety of techniques that have been used to examine the chemistry of capping ligands,16,21 NMR spectroscopy is especially powerful. NMR spectroscopy is an in situ and nondestructive analysis that provides information about the organic molecules interacting with NCs in solution.13 The in situ nature of NMR spectroscopy is particularly valuable for examining the capping ligands under conditions that parallel reactivity and spectroscopic studies. Diffusionordered NMR spectroscopy (DOSY) and related exchange spectroscopies have been used to investigate the interaction of ligands with NC surfaces15,22 and to extract overall sizes using the Stokes
Einstein relation.15,23 Here, we report analyses of the DDA ligand shell in a series of ZnO NC samples suspended in toluene. A method to quantify capping group densities on ZnO NCs is described. NMR spectroscopic data VOL. 8

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* Address correspondence to [email protected]. Received for review July 2, 2014 and accepted August 17, 2014. Published online August 18, 2014 10.1021/nn503603e C 2014 American Chemical Society

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Figure 1. Illustration of one quadrant of a faceted NC (d = 3.8 nm) with DDA capping ligands. The proportion of strongly bound (black carbon atoms) and weakly bound DDA (gray carbon atoms) is depicted in the proportions measured by NMR spectroscopies, showing the sparse surface coverage by DDA.

implicate a model with strongly bound, weakly associated, and free DDA. The strongly bound DDA ligands cover only 10
20% of the estimated number of surface Zn atoms and account for less than 25% of the maximum coverage predicted for a self-assembled monolayer. Figure 1 is a quantitative illustration of the measured density, roughly to scale. It shows a quarter of a 3.8 nm NC with its roughly 10 strongly bound DDA ligands and slightly fewer weakly bound DDA groups. Thus, these NCs have very incomplete surface shells of capping ligands, which contrasts with the typical view. Annealing the NCs causes an increase in the coverage of the strongly bound ligands and a decrease of weakly associated ligands. Since annealing causes the loss of water, this result supports the hypothesis that strong binding of DDA is blocked by surface protons (hydroxide ligands). RESULTS AND DISCUSSION Characterization of Dodecylamine Bound to ZnO Nanocrystals. ZnO NCs with average diameters (d) between 3 and 7 nm were synthesized at room temperature in multigram batches by the wet chemical method described previously,24 capped with DDA, suspended in toluene and stored at
35
C. The mean ZnO NC size was determined from the position of the first exciton peak25,26 or by transmission electron microscopy (TEM). The total Zn concentration in the solution was measured by inductively coupled plasma-optical emission spectrometry (ICP-OES).17 These data give the concentration of NCs, which was on the order of 0.01
1 mM for the experiments described here. Figure 2 compares a typical 1H NMR spectrum of capped ZnO NCs in toluene-d8 versus DDA alone. For ZnO and similar systems, 1H NMR resonances of ligands in the presence of NCs often appear downfield of the free ligand and are significantly broadened.22,27 Broadening of the NMR ligand signals likely arises from multiple effects, including slow tumbling of the large NCs, decreased T2, VALDEZ ET AL.

Figure 2. 1H NMR spectra (700 MHz) of DDA only (top) and capped ZnO NCs (bottom) in toluene-d8. NC d = 4.3 nm, [NCs] = 6  10
4 M, and the asterisk refers to the residual solvent signal. In the presence of NCs, the R-CH2 signal is broadened and the amine peak is not observed.

Figure 3. Fitting of the aliphatic signals in the 1H NMR spectrum of ZnO/DDA solutions (the same data as in Figure 2). The experimental spectrum is shown in black, the blue traces are individual peak fits, and the red is the sum of the fits.

heterogeneity of the NCs in size, shape and surface properties, and the relatively high viscosity of the solutions.13 In the presence of ZnO NCs, the signal at 0.4 ppm for the DDA NH2 group disappears, the R-CH2 peak is broadened and shifted downfield (to ∼2.7 ppm), and the methyl and composite methylene peaks (
CH3, 0.9 ppm;
(CH2)10
, 1.3 ppm) broaden and develop downfield shoulders (Figure 2). The 1H NMR resonance for the internal methylene protons (
(CH2)10
) in ZnO/DDA solutions is well modeled as the sum of two peaks: one broad and more downfield (1.37 ppm), the other sharper and slightly upfield (1.31 ppm) (Figure 3). The
CH3 resonances are also well modeled as two peaks, and essentially the same ratio of broad to sharper peaks is obtained. On the basis of the peak positions and widths, and the DOSY experiments described in detail below, the broad downfield signals are assigned to DDA strongly bound to the NCs. With larger NCs, the distinction between the broad and sharp signals is even more pronounced. DOSY and Nuclear Overhauser Effect spectroscopy (NOESY, see Supporting Information) confirmed that the broad downfield peaks correspond to DDA bound to the NCs. A 1H DOSY NMR spectrum of the ZnO NCs in toluene-d8 is shown in Figure 4. DOSY is a pseudotwo-dimensional technique, displayed with the onedimensional 1H NMR spectrum on the horizontal axis and diffusion coefficient (D) along the vertical axis. VOL. 8

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D ¼

kB T 6πηRH

(1)

Figure 4. Typical 1H DOSY NMR spectrum of ZnO NCs with DDA capping ligands in toluene-d8. The data processing was done with DOSYToolbox 1.5, using Local Covariance Order Diffusion-Ordered Spectroscopy (LOCODOSY) for better resolution of the overlapping peaks.28 Inset shows just the DDA methyl peak (dotted red box) to highlight the differences in chemical shift between the strongly bound DDA and the faster diffusing DDA.

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In the DOSY spectrum, the DDA signals between 0.5 and 2 ppm are split into two separate diffusion regimes (Figure 4, top). The D for the more slowly diffusing DDA peaks, 1.1(4)  10
10 m2 s
1 (errors reported in parentheses are twice the standard deviation from replicates), corresponds to the broad downfield resonance shown in Figure 3. This D implies a hydrodynamic radius of 7 nm by eq 1. This was confirmed in separate experiments by referencing to the D of ferrocene in the same solution (RH(Cp2Fe) = 0.3 nm23). The diameters obtained by the DOSY experiments are roughly the NC diameter size plus two ligands (see Supporting Information), which suggests that the NCs are not significantly aggregated. The assignment of these slowly diffusing DDA molecules as strongly bound is consistent with literature precedent,15 and is supported by a dilution experiment. Dilution of a sample of colloidal NCs by a factor of 5 led to no change in the strongly bound DDA groups per NC (vide infra). The diffusion peaks at ∼1  10
10 m2 s
1 are thus assigned as strongly bound ligands, and this value is termed Dbound. This apparent diffusion constant did not vary between batches of NCs that contain varying amounts of free DDA. These results indicate that strongly bound DDA do not exchange with free DDA on the NMR time scale (Table 1). In contrast, the D values for the faster diffusing DDA peaks vary considerably from batch to batch, between 4.9 and 9.7  10
10 m2 s
1. To study this variability, extra DDA (65 per NC) was added to a solution of asprepared NCs, and a new DOSY NMR spectrum was recorded (Table 1, NC batch B*). With excess DDA, the diffusion peak shifted to larger D values. This observation indicates that the second diffusion peak corresponds to species in a rapid equilibrium with free DDA, resulting in an averaged diffusion coefficient. This result implies that this diffusion constant corresponds

D can be related to the hydrodynamic radius RH via the Stokes
Einstein relation eq 1, where kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the solution.15,16 D is inversely proportional to RH because larger species move more slowly in solution. Trimethoxybenzene (TMB) was added to the solution as an internal standard, and analysis of its resonances at 3.3 and 6.1 ppm gave a diffusion constant of 1.6  10
9 m2 s
1, which is typical for a small molecule.23

1

TABLE 1. Quantification of Dodecylamine Ligands on a Variety ZnO NCs Batches Using H and DOSY NMR Data NC batch NC diametera Dboundb

A B B* g B** g C D E Fannealed Gannealed

3.6(4) 3.8(4) 3.8(4) 3.8(4) 4.3(4) 4.7(5) 6.5(20) 9.2(10) 9.5(10)

0.9(1) 0.9(2) 0.9(2) 0.9(1) 1.3(3) 1.1(4) 0.9(1) 1.1(10) 0.8(2)

Dmixc

χboundd

χweake

χfreee

surface area (nm2)f

7.1(1.4) 4.9(9) 9.3(14) 5.9(6) 6.8(4) 9.7(13) 9.0(12) 12(1) 7.1(14)

0.43(6) 0.47(5) 0.39(15) 0.47(5) 0.34(6) 0.39(4) 0.59(4) 0.51(8) 0.89(12)

0.32(12) 0.38(8) 0.23(2) 0.34(3) 0.39(4) 0.22(10) 0.17(8) 0.10(4) 0.06(4)

0.26(10) 0.15(8) 0.39(16) 0.20(2) 0.27(2) 0.40(14) 0.24(6) 0.39(6) 0.05(3)

40 46 46 46 58 70 131 266 284

total DDA/NC

DDA bound/NC

ligand/nm2 DDA ligand/nm2 free/NC (bound þ weak) (bound)

DDA weak/NC

120(40) 50(20) 40(10) 30(10) 100(30) 40(15) 35(10) 25(15) 140(40) 50(20) 30(10) 60(20) 90(20) 40(15) 30(10) 20(5) 150(30) 55(10) 55(15) 40(10) 220(80) 85(40) 45(10) 90(40) 340(190) 200(100) 60(40) 70(50) 1870(100) 920(80) 200(100) 760(30) 1180(120) 1030(100) 90(70) 60(40)

2.1(2) 1.7(4) 1.8(3) 1.6(4) 1.9(4) 2.0(5) 2.0(11) 4.2(3) 4.0(2)

1.2(2) 0.9(3) 1.1(4) 0.9(3) 0.9(2) 1.2(4) 1.6(8) 3.4(2) 3.6(2)

a Average diameter of the NCs in the batch (values in nm), from the position of the first exciton peak (for d < 7 nm)25 or TEM (for larger NCs), and the error is reported in parentheses as 10% or the standard deviation derived from TEM, respectively. b D values in units of 10
10 m2/s and data are averaged using at least two of each 1D 1H and DOSY experiments. Errors are calculated as two standard deviations from replicates or 10%, whichever was greater. c Dmix is the averaged diffusion coefficient between weakly bound and free DDA. d The mole fraction of strongly bound DDA (χbound) was calculated from fitting of a 1D NMR spectrum. e The mole fractions were calculated using eqs 2 and 3, assuming there are only three different classes of DDA in these solutions with ZnO NCs. Dfree = 1.45  10
9 m2/s for all of the calculations. f Diameter and surface area of the ZnO NCs without capping ligands, calculated assuming a spherical shape. g Batch labeled B* corresponds to batch B plus 65 equiv of DDA per NC. Batch labeled B** corresponds to batch B diluted a factor of 6 to a final concentration of 1.3  10
4 M in NCs.

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Dmix ¼ χweak Dweak þ χfree Dfree

(2)

χfree þ χweak þ χbound ¼ 1

(3)

Combining the data allows the calculation of the absolute number of bound DDA molecules per NC. The mole fractions and the NMR integrations versus an internal standard give the concentration of each class of DDA. The concentration of NCs is given by the total Zn concentration (from ICP-OES) divided by the average number of Zn per NC.11 Remarkably, 3.8 nm ZnO NCs have only 40
50 strongly bound DDA ligands. The number of capping groups per squared nanometer (nm2) of NC surface area, also known as the ligand density, is given in Table 1. These calculations assume a spherical shape for the ZnO NCs. The average VALDEZ ET AL.

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to more than one “type” of DDA, with one of the species interacting with the NCs.29 Therefore, this peak is termed Dmix, the diffusion peak corresponding to the concentration-dependent equilibrium between free DDA and weakly associated DDA. Here “weakly associated” encompasses all of the bound DDA molecules that exchange rapidly with free DDA and are not strongly bound to the NC surface like those with Dbound. The nature of the bonding of the weakly associated DDA could involve Zn2þ sites that are less Lewis-acidic, van der Waals interactions of the hydrocarbon chains with the strongly bound DDA (perhaps with the amine pointing out), and/or hydrogen bonding between the amine and the NC surface. Experiments with annealed nanocrystals discussed below favor a hydrogen bonding interaction with surface hydroxide groups. The NMR data are well modeled using these three different classes of DDA binding to NCs: free, weakly associated, and strongly bound. A similar distinction between different levels of DDA association was presented in the study by Kahn et al.15 They measured approximate rate constants of koff‑strong = 13 s
1 for dissociation of strongly associated DDA and koff‑weak = 25  105 s
1 for dissociation of weakly associated amines.15 This fast desorption rate of weakly associated amine from the NC surface is consistent with the rapid exchange with free DDA in the NMR spectra reported here. Quantification of the Number of Dodecylamine Ligands in Each Regime. One benefit of the averaged Dmix in this two-state model is the ability to determine the molar fractions of free and weakly associated DDA. For example, in a system with 100% free DDA (i.e., no weakly associated), Dmix would be equal to Dfree. The two unknown mole fractions χweak and χfree are determined from the set of two equations below, using the fraction of strongly bound DDA (χbound) calculated from integration of a 1D 1H NMR spectrum (Figure 3), and assuming that the unknown Dweak is equal to Dbound.22

Figure 5. Number of DDA per NC versus NC diameter (left), and the number of ligands per nm2 versus NC diameter (right). Errors are calculated as two standard deviations from replicates or 10%, whichever was greater. Black dotted line indicates SAM coverage for single crystal ZnO (4.2/nm2),30 and green dashed line indicates the coverage predicted with one DDA per calculated surface Zn (∼7.7/nm2, average calculated from Table 2).

ligand densities are remarkably constant across all as-prepared batches of NCs and sizes (Figure 5). The average ligand density of strongly bound DDA only is 1.0(3)/nm2, and the density of both strongly bound and weakly associated DDA is 1.9(4)/nm2. Different values are found for annealed NCs, as discussed below. Comparison of Ligand Surface Density to Surface Cation Estimates or SAMs. The significance of the observed surface ligand densities is revealed by comparison with the maximum surface coverage. To assess the maximum surface coverage, one can (1) estimate the number of available surface Zn binding sites or (2) compare to maximum coverage of similar ligands in the self-assembled monolayer (SAM) literature. There are a number of published methods to estimate the number of surface ions. The values given in Table 2 use the method of Kuno et al.19 that counts any Zn atom within 2.0 Å of the surface (one Zn
O bond length31). The many other published methods all give higher numbers of surface Zn ions (as discussed in the Supporting Information), so the values in Table 2 are lower estimates of surface Zn per NC. Including both the weakly associated and strongly bound ligands, the surface Zn sites are 25(6)% covered, independent of NC size and concentration. Considering only strongly bound DDA ligands gives 15(5)% coverage of surface Zn. Thus, a quarter or less of the calculated Zn surface atoms are bound to DDA according to surface cation calculations. It is also informative to compare capping densities to maximum coverages achieved in SAMs. In this literature, the maximum coverage or saturation is often defined as “the highest possible packing of molecules”, which generally does not correspond to one ligand per surface Zn atom.32 For single crystal ZnO(1010) surfaces, Chen et al. found that although DDA did not form a stable monolayer, similarly sized or larger thiols and phosphonic acids formed monolayers with surface coverages of 4.2 and 4.4 molecules per nm2, respectively.30 In the gold nanoparticle literature, Hinterwirth et al. observed higher ligand densities on nanoparticles than VOL. 8

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NC batch

NC diameter (nm)

surface Zn atomsa

% surface Zn with strong þ weak DDA

% surface Zn with strong DDA

Zn surface atoms without DDAb

A B C D E Fannealed Gannealed

3.6(4) 3.8(4) 4.3(4) 4.7(5) 6.5(20) 9.2(10) 9.5(10)

300 340 440 540 1030 2140 2280

30% 22% 25% 24% 26% 58% 49%

18% 12% 12% 15% 20% 48% 45%

210(30) 270(30) 330(20) 410(60) 760(100) 900(200) 1160(50)

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a

TABLE 2. Calculation of Surface Coverage Using the Number of Zn Unit Cells per NC

a Calculated assuming that all Zn atoms within 2 Å of the surface (one Zn
O bond) can be considered a surface Zn, following Kuno et al.19 b This estimate is conservative because it assumes both strongly bound and weakly associated DDA occupy Zn sites, and assumes fewer surface atoms than most estimates in the literature (see Supporting Information).

on a flat surface, up to 6.3 per nm2, versus the theoretical 4.6 per nm2 maximum coverage for a flat surface.32 These authors suggest the higher coverage arises from the existence of edges and corners in a spherical nanoparticle. Thus, taking the nonpolar ZnO(1010) surface as representative of the various nanocrystal surfaces, the small DDA capping group should provide a maximum surface coverage of >4.2 per nm2 on ZnO, and likely higher due to the small footprint of DDA and the edges and corners inherent to a quasi-spherical NC. Considering strongly bound DDA ligands only, the ligand density of ZnO NCs reported here, 1.0(3) per nm2, is less than 25(8)% of the 4.2 per nm2 lower estimate anticipated for a ZnO SAM. If the weakly associated DDA molecules are also counted, the total ligand density of 1.9(4) per nm2 rises to only 45(10)% of that anticipated for a SAM. Reported surface densities for anionic ligands such as oleate and TOPO-derived anions, on chalcogenide and phosphide NCs, span values from 0.57 to 5 per nm2.13,33
37 These have been determined using 1H NMR spectroscopy, thermogravimetric analysis (TGA), or X-ray photoelectron spectroscopy (XPS). It is important to distinguish neutral (“L-type”) capping ligands, like DDA, from anionic ligands such as carboxylates or phosphonates (“X-type”). Dissociation of X-type ligands typically require stoichiometric protons, forming HX, and the activity and concentration of these protons may be a key factor for ligand coverage.38 Comparisons between ZnO and chalcogenide NCs are complicated not only by the different ligand types but also by different unit cell sizes (unit cell volumes: CdSe, 112.2 Å3; ZnO, 47.6 Å3),39 which imply different cation surface densities. In sum, the surface coverages of as-prepared ZnO nanocrystals by strongly bound and weakly associated DDA are quite low compared to the typical image of a fully covered NC, by comparison to both surface cation estimates and the maximum coverage estimated for SAMs. Origin of Low Capping Group Coverage. The low surface coverage could have a number of origins: sterics, inaccessibility of Zn sites, or blockage by other species. We discuss each of these possibilities in turn. VALDEZ ET AL.

The density of capping ligands on NCs has been examined theoretically, as in Bullen and Mulvaney's modeling of a spherical NC with ligands represented by cones.40 They estimated maximum surface coverage for large ligands such as trioctylphosphine oxide (TOPO) to be limited by steric bulk to only 30% of Cd þ Se surface atoms for a 3.4 nm NC. Katari et al. found these estimates to agree with experimental X-ray photoelectron spectroscopy (XPS) coverage estimates of TOPO on CdSe.20 However, terminal, linear aliphatic amines such as DDA have a much smaller “swept-out” cone radius,40 which should allow binding to all surface metal atoms even on a flat surface (see Supporting Information). X-ray structures of smallmolecule μ-oxo bridged Zn clusters with amine ligands also suggest that sterics should not limit DDA binding density.41 Therefore, sterics are unlikely to play a part in the low surface coverage. The low observed DDA coverage may also be caused by blockage of Zn sites by other surface species or “ligands” for Zn. The likely candidates are DMSO, hydroxide, or ethoxide, since the NCs are formed in a hydrolytic process that involves ethanol and DMSO.24 Unfortunately, such surface species are not directly observable by 1H NMR spectroscopy. Alcohols are known to dissociate on the polar ZnO(0001) face to form stable alkoxide intermediates.42 In support of this hypothesis, added ethanol does reversibly interact with the NCs as judged by a change in the 1H NMR chemical shift of the added EtOH. Treating NCs with ethanol-d6 produced some CH3CH2OD, by 1H NMR spectroscopy, but only a very small amount, 7 nm). The error is reported as 10% or as the standard deviation derived from a Gaussian fit to the histogrammed TEM, respectively. Optical spectra were acquired using a Hewlett-Packard 8453 diode array spectrophotometer. NMR Spectroscopy. All samples were prepared under N2 in a glovebox. 1D 1H NMR spectra were obtained on Bruker AV300, AV301, DRX499, AV500, AV700 or AV800 (with a cryogenically cooled probehead) spectrometers, and the data were processed using MestReNovaª version 7.1.2. DOSY data were collected on the DRX499, AV700, and AV800 spectrometers equipped with TBI Z gradient probe heads. The DOSY data were processed with “DOSY Toolbox” version 1.5 using Local Covariance Order Diffusion-Ordered Spectroscopy (LOCODOSY) to more effectively resolve overlapping peaks28 and the results were superior to those obtained from DOSY processing with MestReNova or TopSpin. Chemical shifts are reported relative to TMS by referencing the residual solvent. 1H NMR spectra were recorded with a 15 s delay between scans to allow full relaxation of the DDA and methyl signals of trimethoxybenzene (TMB), which was added to the NMR samples as a concentration standard. The T1 was measured for the methyl signals of TMB and were found to be approximately 2 s. In the DOSY experiments, the pulse program used was ledbpgp2s, and the gradient shape SINE.100 or SMSQ10.100. The gradient duration δ and the diffusion delay Δ were optimized for each sample by comparing one slice of signals at 95% gradient strength and 5% strength (using 1D pulse program ledbpgp2s1d). Values were chosen which led to approximately 90% loss of the initial intensity of the peaks of interest at highest gradient, and were typically δ = 1
3 ms and Δ = 100
200 ms. The eddy current compensation was set at 5 ms. In the full DOSY experiment, the amplitude of the sinusoidal gradient pulses was varied exponentially in strength from 5 to 95% of the maximum strength in 16 or 32 steps. Representative data are shown in Supporting Information Figure S1. Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: Additional NMR spectroscopy data, calculations, and figures. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. C.N.V. gratefully acknowledges support from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100, and the ARCS Foundation for a fellowship. A.M.S. is funded by

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complex shell of DDA ligands. This provides one explanation for the facile charge transfer reactions of these ZnO NCs,11,47,48 especially proton-coupled

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